1. Field of the Invention
The present invention relates to a photopolymerizable composition which polymerizes and cures upon exposure to visible light on account of a new polymerization initiator contained therein. More particularly, it relates to a photopolymerizable composition suitable for use in dental applications.
2. Description of the Prior Art
Recently, photopolymerizable resins which cure upon exposure to ultraviolet rays and visible light are replacing thermosetting resins which cure upon heating in many application areas such as paints, printing inks, adhesives, coating materials, and dental filling materials.
Heretofore, the photopolymerizable dental composite resin has been of UV curable type or visible light curable type. The one of UV curable type has a disadvantage that curing by irradiation does not reach as deep as required, and a part of it remains uncured at the innermost recess of the tooth cavity. It has another disadvantage of being poor in adhesion to the tooth. These disadvantages have been overcome to some extent with the advent of visible light curable composite resin. Nevertheless, it does not cure completely when a deep tooth cavity is filled by a single application, and filling has to be done twice. It is considered that these drawbacks are attributable to the insufficient curing performance of the photopolymerization initiator.
The conventional polymerization initiators for photopolymerizable compositions are disclosed in U.S. Pat. Nos. 4,071,424 and 4,459,193. The former discloses the use of .alpha.-diketone and amine as the photosensitizer and accelerator, and the latter teaches the combined use of fluorenone or .alpha.-diketone and an organic peroxide and optionally an amine. These conventional polymerization initiators have some disadvantages. That is, the .alpha.-diketone/amine type and the fluorenone/or .alpha.-diketone/organic peroxide type are not sufficient in curing rate and cure depth, and consequently they are not of practical use. On the other hand, the fluorenone/or .alpha.-diketone/organic peroxide/amine type has a high curing rate but is so poor in storage stability that it cannot be packed together with a resin composition. The poor storage stability may be overcome by packing amine separately from .alpha.-diketone/or fluorenone/peroxide. The separate packs are combined together just before use. But this is troublesome and undesirable.
Japanese Patent Laid-open No. 130693/1979 discloses a UV polymerization initiator composed of benzil and N,N-dialkylaminobenzaldehyde. This one is not of practical use either because of its poor storage stability and its tendency toward discoloring the cured product.